A light emitting diode (“LED”) is an illuminating device that converts electrical current into light. The luminosity of an LED is mainly determined by the magnitude of the driving current. Hence, to obtain a stable luminous output without flicker, LEDs should be driven by a constant-current source.
General lighting products are usually powered by an AC mains voltage of 110 or 220 VAC at 50 or 60 Hz line frequency. Single-stage power factor correction (“SSPFC”) drivers are commonly used to convert AC line voltage into DC voltage for driving these LEDs. This is because an SSPFC driver is a simple circuit that achieves both voltage conversion and power factor correction. However, the input power to the SSPFC varies in correspondence to the AC mains voltage due to power factor correction. The input power ripple induces an output voltage ripple and output current ripple. Consequently, using an SSPFC driver to power LEDs directly leads to light flickering at twice the line frequency. Increasing the output capacitance of the SSPFC driver is a direct approach to reduce the output power ripple. However, the value of output capacitors is limited by factors such as size, cost and lifetime. Other solutions exist, but such solutions lead to a reduction in efficiency, increase in cost and more complex designs.